This invention relates to transducers and, more particularly, to an inductive transducer of the type comprising first and second relatively movable members, the first relatively movable member having a plurality of windings and the second relatively movable member having a winding.
Inductive transducers of the above-type have been known for many years, as exemplified by U.S. Pat. Nos. 2,799,835; 2,915,722; 2,942,212; 3,090,934; 3,148,347; 3,441,888; 3,668,587 and 3,673,584. These transducers have been used in both position measuring systems, as exemplified by U.S. Pat. No. 3,191,010, and in conjunction with servo control systems to control the direction and speed of movement of a movable element, as exemplified by U.S. Pat. Nos. 3,839,665 and 3,954,163.
U.S. Pat. No. 3,191,010 discloses a phase-sensitive inductive transducer wherein a first input signal is applied to one of the windings of the first relatively movable member and a second input signal is applied to another of the windings of such member. An output signal is thus developed by induction on the winding of the second relatively movable member. Typically, the first and second input signals are sinusoidal in nature of substantially identical frequency and peak amplitude and are phase-displaced by a predetermined amount (e.g. 90.degree.) since the two windings of the first relatively movable member are displaced in space phase by such predetermined amount (e.g. in space quadrature). Accordingly, then, the output signal induced on the winding of the second relatively movable member is substantially constant in peak amplitude and variable in phase during relative movement of the first and second relatively movable members. By appropriately demodulating this output signal, a position signal may be derived that is periodic in nature in response to relative movement.
Copending U.S. Application No. 670,463 filed on Mar. 25, 1976, now Patent No. 4,059,789 in the name of Kenneth W. Cocksedge and copending U.S. Application No. 737,972 filed in the names of James O. Jacques and Robert D. Carlson, both assigned to the assignee of the present invention, disclose phase-sensitive transducer apparatus of the type above-described as used in conjunction with servo control systems.
In most contemporary servo control systems utilizing a position signal as derived from an inductive transducer apparatus, movement of the controlled movable element is generally detected by sensing zero-crossings, or null points, of the position signal. In the case of a disc drive, for example, the null points can be used to define track crossings wherein one of the two relatively movable members is fixed and the other is connected to the carriage of a read/write head. It is important to derive a position signal that varies as linearly as possible in response to movement of the head relative to the tracks recorded on the disc surface for proper head positioning. Accordingly, it is desired that the induced output signal on the second relatively movable member change in phase as linearly as possible in response to relative movement.
Many prior art inductive transducers of the phase-sensitive type are sensitive to relative movements between the two relatively movable members in directions other than the requisite translational direction. There are five degrees of relative movement other than the requisite translational movement, i.e. two other in translation and three in rotation. Relative motions occurring in any one or more of these five unwanted degrees might result in variances in the inductive coupling between the windings of the two relatively movable members. Such variances may cause an unwanted shift in the phase of the output signal which could destroy the desired linearity of phase change, as discussed above.
Capacitive coupling has traditionally presented a problem in inductive transducers. Capacitive coupling effects could alter the apparent inductive coupling between the windings of the two relatively movable members, if not properly compensated for. As indicated above, such alteration may cause an unwanted shift in the phase of the output signal, thereby destroying the linearity of a position signal derived therefrom.
It would be desirable, therefore, if the phase of the output signal from a phase-sensitive transducer could be substantially insensitive to relative movements in at least some of the five unwanted degrees of motion. It would further be desirable if the effects of capacitive coupling between the windings of the two relatively movable members could be minimized and/or compensated for in order to maintain a substantially linear relationship between the relative movement of the two relatively movable members and the change in phase of the output signal from the transducer.